The gases in air, such as in particular oxygen and nitrogen, are very important industrially. At present, one of the non-cryogenic techniques used for producing these gases is the technique referred to as PSA (pressure swing adsorption), which encompasses not only PSA processes proper, but also similar processes, such as VSA (vacuum swing adsorption) or MPSA (mixed pressure swing adsorption) processes.
According to this PSA technique, when the gas mixture to be separated is air and the component to be recovered is oxygen, the oxygen is separated from the gas mixture using preferential adsorption of at least nitrogen on a material which preferentially adsorbs at least nitrogen and is subjected to cycles of given pressure in the separation zone.
The oxygen, which is adsorbed little or not at all, is recovered at the outlet of the separation zone; it has a purity, in general, higher than 90%, or even 93%.
More generally, a PSA process for the non-cryogenic separation of a gas mixture comprising a first compound which is adsorbed preferentially on an adsorbent material, and a second compound which is less preferentially adsorbed on the adsorbent material than the first compound, with a view to producing the second compound, cyclically comprises:
a step of preferentially adsorbing at least the first compound on the adsorbent material, at an adsorption pressure referred to as the "high pressure", with recovery of at least some of the second compound produced in this way; PA1 a step of desorbing the first compound trapped in this way by the adsorbent, at a desorption pressure which is lower than the adsorption pressure and is referred to as the "low pressure"; PA1 a step of recompressing the separation zone comprising the adsorbent, by progressively changing from the low pressure to the high pressure. PA1 a) at least one mother liquor containing lithium salts having a molar purity in excess of 98% is percolated through at least one bed of zeolite particles, the mother liquor making it possible to obtain a limit exchange factor (LEF) for the zeolite particles of between 90 and 100%, PA1 b) the percolation of the mother liquor is stopped when an amount of mother liquor has been used which is necessary to obtain a mean real exchange factor (REF) such that: REF=LEF-2%.+-.1% PA1 c) the lithium-exchanged zeolite particles obtained in step b) are recovered. PA1 the mother liquor has a purity of from 98% to 99.9%, preferably from 99.3% to 99.8%. PA1 the LEF is between 93% and 99%. PA1 the REF is between-90% and 98%. PA1 the ion exchange is carried out at a temperature of between 700.degree. and 140.degree. C., preferably of the order of 100.degree. C. PA1 the zeolite is a faujasite, preferably an 5 X zeolite having an Si/Al ratio of the order of 1. PA1 the first compound is nitrogen and the second compound is oxygen; and an oxygen-rich gas flow is produced, that is to say one generally comprising at least 90% to about 95% of oxygen. PA1 the high pressure for adsorption is between 10.sup.5 Pa and 10.sup.7 Pa, preferably of the order of 10.sup.5 Pa to 10.sup.6 Pa, and/or the low pressure for desorption is between 10.sup.4 Pa and 10.sup.6 Pa, preferably of the order of 10.sup.4 Pa to 10.sup.5 Pa. PA1 the feed temperature is between 10.degree. C. and 80.degree. C., preferably between 25.degree. C. and 60.degree. C.
However, it is known that the separation efficiency for a gas mixture, such as air, depends on many parameters, in particular the high pressure, the low pressure, the type of adsorbent material used and its affinity for the compounds to be separated, the composition of the gas mixture to be separated, the adsorption temperature of the mixture to be separated, the size of the adsorbent particles, the composition of these particles and the temperature gradient set up inside the adsorbent bed.
At present, although it has not been possible to determine a general behaviour law, given that it is very difficult to relate these various parameters to one another, it is also known that the nature and properties of the adsorbent have an essential role in the overall efficiency of the process.
Currently, zeolites exchanged with metal cations are the adsorbents most widely used in PSA processes.
Such zeolite particles contain mono-, di-and/or trivalent metal cations, for example cations of alkali metals, alkaline-earth metals, transition metals and/or lanthanides, incorporated during the synthesis of the zeolite particles and/or inserted subsequently by an ion-exchange technique, that is to say, in general, by bringing unexchanged zeolite particles or raw zeolite into contact with a solution of one or more metal salts comprising the cation or cations to be incorporated into the zeolite structure, and subsequently recovering the- particles of exchanged zeolite, that is to say zeolite containing a given quantity of metal cations.
The proportion of metal cations introduced into the zeolite structure, relative to the total exchange capacity, is referred to as the exchange factor, which is between 0 and 100%.
At present, the adsorbents most widely used in processes of the PSA type for separating gases, in particular air, are zeolites, in particular of the X or LSX type, highly exchanged, in general to more than 80% or even to more than 95%, with cations of very expensive metals, such as in particular lithium cations. Such zeolites are in particular described in documents EP-A-486384, EP-A-606848, EP-A-589391, EP-A-589406, EP-A-548755, U.S. Pat. No. 5,268,023, EP-A-109063 and EP-A-760248.
In the particular case of lithium, the lithium factor is the proportion of lithium cations which are associated with the aluminum in tetrahedral position contained in the zeolite phase of the particle and associated with exchangeable cations; this factor can be expressed by the Li/Al ratio.
Thus, documents U.S. Pat. No. 4,859,217 and 5,268,023 describe X or LSX (low-silica X) zeolites whose nitrogen adsorption capacity increases linearly as a function of the lithium-exchange factor for Li/Al ratios in excess of 80%. In other words, according to these documents, the best lithium exchange factor is the highest factor which can be obtained, namely 99% to 100% according to the examples given in U.S. Pat. No. 4,859,217.
Furthermore, documents U.S. Pat. No. 5,174,979 and U.S. Pat. No. 5,413,625 describe an X or LSX zeolite having an Li/alkaline-earth metal ratio of between 95:5 and 35 50:50. In order to achieve the maximum lithium-exchange factor, the zeolites described in these documents are prepared with heavy consumption of lithium salts, that is to say consumption of between 6 and 12 times the stoichiometric amount needed for complete exchange of the exchangeable cations which are associated with the aluminum of the zeolite phase.
However, contrary to what the prior art suggests, in the zeolite phase of the adsorbent particles there are a certain number of cations other than sodium and lithium, which originate in particular from the synthesis reagents and/or the binder used for manufacturing the particles.
The inventors of the present invention have demonstrated that these other cations are, in general, more difficult to exchange than the sodium cations.
In other words, substitution of these cations with lithium cations is relatively difficult and therefore causes overconsumption of the mother liquor containing the sodium salts, which results in a significant increase in the manufacturing costs of the adsorbent particles.
The consequence of this is that the real exchange factor (REF) which is obtained after ion exchange is not 99% or 100%, as suggested by the prior art, in view of the presence of these cations other than lithium and sodium in the zeolite phase.
It is therefore far preferable, in particular from the point of view of manufacturing costs, to manufacture zeolite particles having a real exchange factor (REF) lower than the maximum exchange factor described by the prior art.
However, for each mother salt containing lithium salts, there is a limit (LEF) or maximum exchange factor which can be obtained for the zeolite particles subjected to an ion-exchange process using this mother liquor.